Do Black Swallowtail Butterflies Migrate? (No, and Here Is What They Do Instead)

Do Black Swallowtail Butterflies Migrate? (The Science)

Monarchs fly south; Swallowtails freeze in place

Every year from late August through October, gardeners across North America find plump black swallowtail caterpillars chewing dill, fennel, parsley, and carrot tops. When these caterpillars pupate into chrysalises and fail to emerge after the standard 10 to 14 days, many people assume the adults have already migrated or died.

In reality, Papilio polyxenes is a strictly temperate resident species across eastern and central North America. Adult butterflies lack the fat reserves and aerodynamic morphology required for trans-continental migration. Instead, evolution equipped the swallowtail pupa to withstand deep snow, howling winds, and sub-zero temperatures attached to tree bark, wooden fences, and garden stems.

What Holds the Chrysalis Shut: Daylength

The trigger is light, not cold

Larval eyes (stemmata) measure the daily ratio of light to darkness. Above roughly 14.5 hours of daylight, in June and July, larval neurohormones signal direct development and the chrysalis opens in 10 to 14 days. Below about 13.5 hours, in September and October, the caterpillar brain stops producing prothoracicotropic hormone (PTTH), which halts adult wing and organ differentiation and seals the chrysalis in dormancy — before the cold arrives at all. That is why an autumn chrysalis sits for 5 to 7 months rather than two weeks, and why warmth alone does not open it.

What Keeps It From Freezing: Glycerol

Biological antifreeze in the hemolymph

A dormant pupa still has to survive ice storms without its cells rupturing. Once diapause is triggered, the swallowtail synthesizes high concentrations of polyols — mainly glycerol and sorbitol — in its hemolymph, the insect equivalent of blood. These cryoprotectants depress the supercooling point of its body fluids below -20°F (-29°C), which is what prevents the intracellular ice crystals that would otherwise tear cell membranes apart.